Traditional process-control systems normally comprise three hierarchical levels. There is, typically, the first (lowest) level of the field devices, for instance, sensors and actuators, which represent the primary interface with the process to be controlled. The second level is constituted by control devices (controllers, PLCs, etc.), which on the basis of information received from the field devices, decides in real time which actions are to be carried out on the actuators in order to achieve given control targets, such as, for example, maintenance of given process variables within pre-set values. The control devices can then communicate with a further level of supervision, in which one or more consoles enable the operator to display the state of the process and possibly issue commands for carrying out manoeuvres, such as activating or de-activating part of the plant or system or setting the value that one or more variables present in the system must reach and maintain.
The field devices are usually physically distributed in the plant. Typically, amongst field devices it is typically possible to number sensors, i.e., devices that can measure one or more process variables (pressure, temperatures, flow-rate, etc.), and actuators, i.e., devices that can act on the process to modify the behaviour thereof (motors, valve positioners, switches, etc.). Note that there exist other types of sensors or actuators used in process-control devices, for example, sensors for measuring the internal temperature in a field device, but in what follows by sensor or actuator is meant specifically just one sensor or actuator directly interfaced with the process, i.e., a sensor that measures one of the process variables or an actuator that acts directly on the process.
In traditional control systems, communication between the field devices and the control devices occurs by means of direct connection between each field device and a control device.
Recently, there have been introduced communication systems that have enabled introduction of a different architecture of the control system. In particular, reference is made here to the communication system defined by the Fieldbus Foundation protocol (in what follows referred to simply as Fieldbus). Using this protocol, a communication network can be obtained between field devices (Fieldbus network), in which said devices exchange directly information without necessarily passing through the second hierarchical level (control level). Rather, the two highest hierarchical levels (control level and supervision level) can advantageously also be connected to the Fieldbus network, exchanging information, on principle, with all the devices connected to said network.
The Fieldbus protocol has introduced a series of technological innovations that enable an increase in the functional capacities of the units (transmitters, actuators). For the present description, just two characteristics/concepts are considered: Function Blocks and Backup LAS (Link Active Scheduler).
The Function Blocks can be considered as parts of software that carry out functions of different nature, such as: measurement algorithms, calculation or control algorithms, input selectors, etc. They are, in general, means for logico-mathematical processing of information.
The concept of Function Blocks enables availability of units, which, in addition to the typical functions for measuring the quantities that they are designed to measure (pressure, temperature, etc.), may contain functions for more general control operations not necessarily dependent upon or linked to the primary function of the device itself.
For example, a temperature transmitter may contain blocks having functions of the PID, mathematical, signal-characterizer type, etc., which are not necessarily linked to the measurement of temperature.
The fact of having transmitters with these different functions “on board” enables the process control to be performed directly between the devices of the field themselves (distribution of control in the field), instead of it being performed in the context of the central-system controller (HOST), as is the case with traditional instrumentation.
In addition, the above capability of Fieldbus units affords considerable flexibility in the control strategy because a Function Block contained in a transmitter could be used within the control of a process in which the transmitter itself does not contribute with its own measurement but just offers a function of which it is the container.
When in a Fieldbus network the Function Blocks are used, it is necessary, in the stage of configuration of the control system, to specify, for each input variable of a Function Block, from which other Function Block said variable comes. In practice, for example, if the Function Block FB1 contained in the device A needs to use the variable 7 generated by the Function Block FB2 contained in the device B, in the configuration stage, it will be necessary to specify that one of the “inputs” of the Function Block FB1 is “logically connected” to the output 7 of the Function Block FB2.
Note that, on the basis of this architecture, in order to recognize what a given variable present on the network refers to, it is necessary to specify in which device said variable has been produced and, more specifically, in which Function Block of the device in question. This can be done in a unique way using the Tags, i.e., alphanumeric strings which are associated to each device and to each Function Block.
The communications and the execution of the Function Blocks must be regulated and scheduled organically to achieve ‘determinism’ in the communication. For example, the one and the same measurement of pressure must be acquired/sampled always at the same instant within a control loop (Macrocycle), hence at a constant rate, and, consequently, it must be transmitted only when it has actually been processed and updated with a new value. The same applies to all the variables that are results or outputs of processing of the Function Blocks.
The above scheduling function is carried out by the LAS (Link Active Scheduler) or bus arbiter. LAS is a function of co-ordination of a Fieldbus network. It is a sort of master activity which manages the Fieldbus activity and may be redundant on one and the same communication network. Normally, the Primary LAS resides in devices having resources, in terms of capacity and speed of calculation, superior to those of a transmitter. Such devices may be, for example, a Linking Device or the central control system itself, and carry out scanning/scheduling of the functions and communications for the process control, irrespective of the fact that these communications or functions are carried out in the transmitters or in the controller of the system itself.
In a standard configuration of this type, where the LAS is active in the control system, there is the possibility of monitoring everything that takes place during the process on the operator's console. There can be displayed the various process variables measured by the various transmitters, as likewise the position of the valve, etc.
A further concept known in the field of Fieldbus networks is the LAS function as a function that can be carried out also by the Fieldbus transmitters. For reasons of safety, management, etc., it is deemed preferable that both devices having LAS capability and devices containing specific Function Blocks for measurement (sensors) or for actuation (actuators) are used just for one of the two functions, i.e., either as Backup LAS or for contributing with their Function Blocks to control loops.
The capability of a transmitter for being activated as a LAS is normally used as Backup of the Primary LAS described above, because the LAS developed within field units or devices is in actual fact a subset of the functions of the primary LAS of a master. Typically, a field device does not present the time-distribution capability for periodic synchronisation of all the Fieldbus units of the network.
A problem of the known art lies in the fact that many transmitters are installed in points of the plant or system that are hard to access. Consequently, even though these devices are equipped with displays of their own, reading of said displays is in fact impossible. This problem becomes particularly serious in the case where a device operating as Primary LAS in a control system fails. In this case, in fact, it is possible to maintain the control loop active only if a second LAS, in this case the Backup LAS, has been provided in the configuration stage. In this case, the transmitter with LAS capabilities, configured for constituting the backup of the system, realizes that the Primary LAS has failed and takes over the job of maintaining active both scheduling of the Function Blocks and scheduling of the communications for the other transmitters of the network that contribute with their Function Blocks to said control loop. This, however, occurs in a blind way from the standpoint of monitoring of what is happening in field. The console is no longer active, and it can only be assumed that the control loop is effectively continuing to operate because it is managed by the Backup LAS active on one of the transmitters of the network.
In the case of failure of the Primary LAS, for displaying the information on the network it would now be possible to use a PC connected to the Fieldbus network and equipped with an appropriate software. This solution presents the disadvantage that a PC is not suitable for fixed in-field installation, in so far as it is not equipped with the necessary protections against dust and water that are typically required of in-field installed devices. In particular, there is typically required a degree of protection IP65 or higher, as defined by CENELEC (European Committee for Electrotechnical Standardization). Alternatively, if the NEMA (National Electrical Manufacturers' Association) directives are used, the device is required to meet a degree of protection NEMA 4 or higher. Current PCs are not pre-arranged for meeting the above requirements. In the case of installation in environments that are dangerous on account of the presence of explosive gases and powders, other types of certification are envisaged, such as, for example, EEx. There fall within this category the certifications EEx d for the explosion test (casing or housing or container of the device) and EEx i for electrical protections and intrinsic safety. In the case in point, for Fieldbus-Foundation instruments, typically reference is made to the two standards ENTITY and FISCO.
A further problem inherent in the known art is that using the Backup-LAS function in a sensor or an actuator, albeit technically possible, risks overloading these devices, which normally, for technical reasons and reasons of cost, are not provided with a high calculation power.
The above limit on the calculation power also implies that, even though also in this case there do not exist a priori theoretical limits, sensors and actuators have in effect a limited capability of execution of Function Blocks.